The present invention relates generally to optical lithography used for fabricating semiconductor devices, and more particularly to optical lithographic phase-shifting masks and a method for fabricating such masks.
Photolithography methods are well-known for producing fine patterns on integrated circuits and other electronic devices. Typically, photosensitive resist material is deposited upon a substrate and a portion of the material is exposed in a predetermined pattern. The pattern is then developed by selective removal or retention, depending upon whether the resist material is a positive or a negative resist.
Exposure of the resist material is typically accomplished by transmitting light, e.g. ultraviolet light, through a mask. Exposure through a mask causes diffraction, image spreading, and/or other interference effects at the boundaries of opaque areas of the masks. Such effects may cause ghost patterns or lobes in the exposed pattern. This is partly because the masks must be at a distance from the resist material during exposure in order to ensure that the size of the pattern at the resist surface is reduced compared to the size of the pattern in the mask. Until fairly recently, these effects were relatively dimensionally small. However, recent increases in the integration density of integrated circuits has pushed minimum feature sizes of patterns such that the effects are now becoming significant.
To improve the clarity of the exposure patterns, phase-shifting masks have been developed to limit the image spreading effects. One type of phase-shifting mask, a rim type phase-shifting mask, assists in limiting image spreading in the exposure of features having a closed shape, such as contacts. Another type is a Levenson-type phase-shifting mask, which assists in limiting image spreading in the exposure of periodically repeated patterns, like parallel lines, such as arrays of parallel conductors.
The fabrication of phase-shifting masks generally has been difficult and expensive due to the need to form extremely small regions having differing optical lengths at the edges of opaque regions. Thus, either patterning must be done within the mask pattern or the opaque regions of the mask must be recessed from the regions of differing optical path length. Examples of known phase-shifting mask fabrication methods can be found in U.S. Pat. No. 5,747,196 (Chao et al.), U.S. Pat. No. 5,633,103 (DeMarco et al.), U.S. Pat. No. 5,532,089 (Adair et al.), and U.S. Pat. No. 5,484,672 (Bajuk et al.). Known phase-shifting mask fabrication methods have had difficulty with ensuring a symmetric exposure of either a printable contact area on the mask or a printable line area on the mask without image spreading effects.
The invention provides a fabrication process for a phase-shifting mask which ensures that a printable contact area or a printable line area is exposed symmetric to an adjacent phase shifting feature.
In one aspect, the invention provides a method of forming a mask. The method includes forming a first layer of material over a substrate and forming an opaque layer overlying the first layer of material. The opaque material layer has at least one opening filled with a second material, the second material residing over the first layer of material and defining areas of the first layer of material which are to be removed. The method also includes using the second material as a mask to remove the areas of the first layer of material, and then removing the second material. The result is a phase-shifting mask which ensures that a printable contact area or a printable line area is exposed aligned to an adjacent phase shifting feature.
In another aspect, the invention provides a method of forming a mask, which includes forming an opaque layer over a substrate, the opaque layer having at least one opening therein filled with a first material, the first material defining areas of the substrate which are to be removed. The method also includes using the first material as a mask to remove the areas of the substrate, and removing the first material. The result is a phase-shifting mask which ensures that a printable contact area or a printable line area is exposed aligned to an adjacent phase shifting feature.
These and other advantages and features of the invention will be more readily understood from the following detailed description which is provided in connection with the accompanying drawings.